Well accepted printing methods in an "office-environment" as e.g. ink-jet printing and electrostatographic methods, are not used as much as would be expected when the convenience of these methods is considered. Most of these printing methods can only partially print continuous tone images and the continuous tone image has to be specially treated (e.g. by a dither method) before the print can be made. In this context, a continuous tone image or contone image is an image containing grey levels, with no perceptible quantisation to them. This drawback has hampered the use of these very convenient printing methods in those imaging areas where it is important to accurately print continuous tone images as e.g. in pictorial photography, medical imagery, etc.
In ink-jet printing, a convenient printing system for use in an office environment, it has been proposed in EP-A-606 022 to use different inks, with different pigmentation and to use the ink with low pigmentation to print the low densities and the ink with high pigmentation to print the high densities. In this technique use is made of ink drops with volumes ranging from 25 to 100 .mu.l in the so called bubble jet based systems, or with volumes in the range of 5 to 10 .mu.l in the so called continuous jet systems. In all cases the images are built up by combining in an appropriate way such drops on the substrate, and although the addressability of each drop typically lies in the range of 300 dpi (dots per inch, or dots per 25.4 mm) to 1200 dpi, the not fully reproducible way the dot spreads and penetrates in the substrate limits the real resolution in the printed image. Hereinafter the resolution of image will be described in dpi, a normal description in the printing business. 1 dpi (one dot per inch) equals 1 dot per 25.4 mm. Further attempts to reproduce continuous tone images using light- and dark-coloured inks have been described in EP-A-606 022 and U.S. Pat. No. 4,860,026.
Electro(stato)graphic methods are evenly well accepted imaging methods in an "office environment" as ink-jet printing since these methods, e.g. electrophotographic copiers, electrographic printers, Direct Electrostatic Printing (DEP), are convenient, fast, clean and do not need liquid solutions or dispersions. Since electro(stato)graphic methods may use solid particles that typically have a particle diameter between 1 and 10 .mu.m as marking particle, it is possible to achieve very high resolution in electro(stato) graphy.
However, most electro(stato)graphical imaging systems, are not intrinsically capable of forming continuous tone and special measures have to be taken to print continuous tone images.
Continuous tone printing in electrophotographic printing by a laser beam is described in the Journal of Imaging Technol., Volume 12, n.sup.o Dec. 6, 1986 on pages 329 to 333 in an article entitled "Electrophotographic colour Printing Using Elliptical Laser Beam Scanning Method". In this article a dot matrix method, combined with pulse-width modulation of the laser beam (to be able to introduce in each dot of the matrix several density levels) and with an elliptical laser beam, is described to achieve a continuous tone reproduction with sufficient resolution and linearity over a tone range of 256 levels. Although with such a printing system quality continuous tone prints can be made, there are still some problems to be addressed. On an electrostatic photoreceptor there is a threshold level of toner adhesion: this means that in the low density areas, where the electrostatic latent image is weak and is situated just above that threshold, the system shows inherently some instability in the low density areas. Also, since the low density areas are printed using very few toner particles, the granularity (in other terms graininess or noise) in the low density areas becomes easily objectionable for high quality prints.
In Patent Abstract of Japan vol. 007 no. 290 (p. 245), Dec. 24, 1983 & JP-A-58/162970 (Hitachi Seisakusho KK), Sep. 27, 1983 a second toner having a same colour and a lower colour density (1.0 black density) is added in a single development station to a first toner (1.8 black density), already present in said single development station, in a 4:1 ratio to obtain a good gradation. The gain in density resolution of the printed image (i.e. having a continuous tone image) is rather limited.
In U.S. Pat. No. 5,142,337 a second toner is used, comprising a mixture of opaque black, opaque white and clear toner. A second toner layer is applied on top of a first toner layer, comprising black toner. In this method also, although image quality is high and very even images can be produced, the gain in density resolution of the printed image (i.e. having a continuous tone image) is rather limited.
In proceedings of the International Congress on Advances in Non-Impact Printing Technologies, San Diego, Nov. 12-17, 1985, no. Congress 5, Nov. 12, 1989, Moore J., pages 331-341, Kunio Yamada et al `Improvement of halftone dot reproducibility in laser-xerography`, the author discusses graininess of the xerographic process, mainly influenced by dot growth.
A method for printing a continuous tone image with electro(stato)graphic printing methods has been described in EP-A-768 577. In this method comprising the steps of partitioning a surface of said substrate into a plurality of disjunctive microdots and applying to at least one microdot at least two types of toner, having substantially the same chromaticity, the intrinsic qualities of electro(stato)graphic printing methods (speed, resolution, cleanness, dry operationable) can be used to print continuous toner images. In that disclosure, the stable printing of high resolution half-tone (continuous tone) images over at least 256 printed (not only addressed) density levels is shown.
The method of that disclosure necessitates the use of different types of toner particles and thus the use of apparatus with different toner stations containing toner particles with the same chromaticity, but having different amounts of colorant. Thus there are several printing stations necessary even for printing monochrome images.
This limits the usefulness of the method of EP-A-768 577 more or less to the use of larger electrostatographic printers. There is thus still a need for a method for stable printing of high resolution half-tone (continuous tone) reproductions of continuous toner images over at least 128 printed (not only addressed) density levels in simple and smaller printers